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The OVER TEMP light will come on if the
dummy loads should begin to overheat. This will
occur if too
much
power
is applied to
the
dummy
loads
for too long, or if
the
air flow
from
the fans
on
the
rear
of the PA81 is restricted. If the OVER
TEMP indicator comes on, remove the power
amplifiers output power
and
allow the PA81
loads
to cool for a minimum of 10 minutes after
the
indicator goes
out.
The DUMMY LOADS are
rated
at 250 watts
per
channel for 5 minutes on
and 10 minutes off,
or
100
watts
per channel
continuous
.
The
open
position provides a quick way to test
the
amplifier
with a no
load
condition, as is
specified by some
manufacturers.
The DC BAL-
ANCE function continues
to
operate in the open
position.
DC Balance
The PA81's DC BALANCE feature trips to re-
move the dummy loads from the amplifier when
it
senses
1 volt DC or greater across either
channel
DUMMY LOAD
input. This protects DC
coupled
amplifiers from avalanching by
open-
ing the
current
path.
When
tripped,
the dummy
loads for both the left and right channel are
interrupted. An LED illuminates to indicate on
which channel the DC offset
occurred.
The
loads remain open until
you
manually reset
them by
pushing the "Reset"
button.
Occasionally,
the DC BALANCE
may
seem to
randomly
trip, staying
set after you
reset it. Two
conditions cause this to
occur.
In some amplifi-
ers this
is
a "normal" condition
and occurs when
you
turn the amplifier on or
off.
(You
will likely
hear a pop
when
the loudspeakers are
con-
nected.)
The second normal condition which may cause
the PA81's DC BALANCE to trip is when music
is applied to the power amplifier.
Music
contains
low frequency beats
which
may instantaneously
appear as DC to the fast-acting PA81 circuits.
NOTE:
When testing and performance testing
power amplifiers, always use a continuous tone
rather than music as
the
input signal. A continu-
ous tone provides a much better test of the
amplifier, and is the /HF/IEEE recommended
test condition.
If either of the above
conditions
occur, reset the
DC BALANCE switch. If it trips immediately you
have a DC offset problem with the channel
indicated. The DC BALANCE function monitors
the dummy load input for DC offset regardless
of where the FUNCTION switch is set.
To use the DUMMY LOAD INPUTS:
1) Connect the PA81 DUMMY LOAD
INPUTS to power amplifier in place of
the loud
speakers.
2) Set the PA81's IHF DUMMY LOADS
switch to match the impedance of the
amplifier.
3) Select
the
desired measurement on the
orange DUMMY LOAD section of the
FUNCTION switch.
4) Apply a continuous tone to the power
amplifier under test.
5) Read the test results on the PA81's
meter, listen to the audio using the
built-in monitor or headphone, or view
the signal on a scope connected to the
TO SCOPE OUTPUT jacks.
AUDIO LINE INPUTS
The AUDIO LINE INPUTS are used to connect
to the LINE OUT jack of audio components. RCA
phono connectors provide simple connection to
tuners, pre-amps, CD players, cassette decks,
turntables etc. The AUDIO LINE INPUTS read
and display the measurement that is selected by
the yellow AUDIO LINE section of the FUNC-
TION switch.
The AUDIO LINE INPUT impedance is 10kohms
which closely matches the Line Out level of most
audio components. (refer to figure 2)
NOTE:
If
you
require a line impedance other
than 10 kohms, follow the procedures given in
the EXTERNAL INPUT section.
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To use the AUDIO LINE INPUTS:
1) Connect the LINE OUT of the audio
device to the PA81AUDIO LINE
INPUTS.
2) Select the desired measurement on
the yellow AUDIO LINE section of the
FUNCTION switch.
3) Use the proper test signal for the
device being tested as specified by its
manufacturer to insure proper test
results.
4) Read the test results on the PA81's
meter, listen to the audio using the
built-in monitor or headphone, or
view
the signal on a scope connected to the
TO SCOPE INPUT jacks.
EXTERNAL INPUTS
The EXTERNAL INPUTS let you signal trace
and measure DC voltages to isolate defective
stages in audio equipment. Their high imped-
ance of 1 Megohm allows you to measure
sig-
nals without loading the circuit. The DC voltage
measuring function is unique to the EXTERNAL
INPUT. Use it as a conventional voltmeter to
detect DC bias problems and make individual
measurements in
stages.
You can use the EXTERNAL INPUTS with an
external
resistor
if you must match to the exact
Line Out impedances of an audio
component
(i
.e. 47 or93
kohm). When
doing so, be
sure
that
you do not interchange the ground and signal
connection between the PA81 and audio com-
PA.81
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Fig. 2: AUDIO LINE INPUTS test low-level signals such as tuners, pre-amps, cassette decks, etc.
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